A construction machine such as a hydraulic excavator includes an engine; a hydraulic pump of a variable displacement type driven by the engine; a hydraulic actuator; and a directional control valve for controlling the flow of hydraulic fluid from the hydraulic pump to the hydraulic actuator based on an operation of an operating member. The construction machine also includes a pilot pump of a fixed displacement type driven by the engine and a regulator device configured to make the displacement of the hydraulic pump variable with using the delivery pressure of the pilot pump.
Thus far, the technique of reducing the load torque necessary for engine start-up has been proposed for the purpose of enhancing the start-up performance of the engine, for example, in a low-temperature environment. In Patent Document 1, the load on the hydraulic pump is reduced by reducing its displacement to a minimum displacement at the time of engine start-up.
While, in Patent Document 2, an unloading valve is provided on the line connecting the delivery side of the pilot pump to a tank, and the load on the pilot pump is reduced by operating the unloading valve to the open position at the time of engine start-up.